Autothermal cracking is a route to olefins in which a hydrocarbon feed is mixed with oxygen and passed over an autothermal cracking catalyst. Combustion is initiated on the catalyst surface and the heat required to raise the reactants to process temperature and to carry out the endothermic cracking process is generated in situ. Such a process is described for example in EP 332289 B; EP 529793 B; EP 709446 A and WO 00/14035.
The autothermal cracking process typically produces a gaseous product stream comprising one or more olefins, hydrogen, carbon monoxide and carbon dioxide. In addition, the product stream will usually also comprise alkanes, such as methane, water, dienes, such as butadiene, acetylenes, oxygenates and aromatic compounds, such as naphthalenes and toluene.
The oxygenates include carbonyl containing compounds, such as carboxylic acids, esters, aldehydes and ketones, especially aldehydes. It is generally desired to remove such oxygenates relatively early in the separation and purification steps which need to be applied to the product stream from the autothermal cracking process, for example, prior to separation of the product olefins from components such as hydrogen, carbon monoxide and carbon dioxide. Although, some oxygenate removal may be achieved by contacting the product stream with a wash water, water alone is not particularly effective at oxygenate removal.
The use of sodium bisulphite to separate oxygenates, such as aldehydes, via complex formation is also well known in the art, and is described, for example, in U.S. Pat. Nos. 3,816,478, 5,157,205 or 6,037,516. However there are problems that must be overcome in applying this to gaseous streams also containing carbon dioxide.
Specifically, bisulphite solutions inherently provide a vapour pressure of sulphur dioxide. The vapour pressure of the sulphur dioxide depends, among other factors, on the pH of the solution. At high pH, the vapour pressure of sulphur dioxide is minimised, but, when carbon dioxide is present in the gaseous stream to be treated, the high pH may cause the carbon dioxide to form carbonates or bicarbonates in the bisulphite solution. Although the carbonate formation can be avoided by operating at lower pH, as the pH of the solution is reduced the vapour pressure of sulphur dioxide in the vapour phase will increase.
The sulphur dioxide may be detrimental to downstream processing steps conventionally required for the treatment of the product stream, such as removal of carbon dioxide and, where present, any catalytic processes for removal of oxygen or the removal of acetylenes.
Thus, an alternative to bisulphite is desired when it is required to remove oxygenates from a gaseous stream also comprising carbon dioxide.